Sure enough, a reliable Griffon vs. sometimes problematic Sabre could be a good idea?
Looking at the charts provided at Mike's site,the 2 stage Griffon has a notable edge in power vs. the Sabre IIA at higher altitudes (and at lower ones, if it's pushed up to 25 lbs/sq in using the 150 grade fuel), eg. 1600 hp achieved at cca 27300 ft vs. 22300 for the Sabre.
How fast it should be? Well, with +9 lbs/sq in boost (some 1850 HP), Tempest II was capable for 430-435 mph @ cca 18000 ft. At 26000 ft, it's some 1400 HP vs. 1700 HP for the Griffon; maybe another 10-15 mph for the Griffon Tempest? The low level performance should be lower vs. the Tempest V, unless the boost is upped to 21-25 lbs/sq in.
What does it have vs. the Spit XIV? All cannon armament, IIRC more rounds per cannon (140 vs 120?), 162 gals of internal fuel (with nose tank) vs. 112 gals, all around canopy. I'm not sure about drop tank carrying ability (2 x 90 gals vs 1 x 90?), so please chip in. Shortcoming is weight.
What does it have vs. LW, a question of a greater importance?